REPORT Review of Generation IV Nuclear Energy Systems REPORT SUMMARY According to many energy foresight studies carried out in the late 1990s, a shortage of uranium can be expected during the 21st century. This spurred the United States Department of Energy (DOE) to set up the Generation IV International Forum (GIF) in 2000, for the purpose of coordinating research and development work aimed at deploying Generation IV nuclear energy systems (reactors and the related fuel cycle facilities), by the second half of this century. Member countries, including France, proposed more than a hundred such systems, of which GIF selected six, considered the most promising in light of various criteria based on the following objectives: continuation of the progress made by Generation III water reactors in terms of competitiveness and safety; more effective use of uranium resources; less radioactive waste, especially high-level, long-lived waste; greater protection against malicious acts and the diversion or theft of nuclear materials. The six systems selected by GIF are: Sodium-cooled Fast Reactors (SFR); Very High Temperature Reactors (VHTR); Gas-cooled Fast Reactors (GFR); Lead-cooled or Lead-Bismuth Eutectic (LBE) cooled Fast Reactors (LFR); Molten Salt Reactors (MSR); SuperCritical Water Reactors (SCWR). GIF selected several fast neutron spectrum systems as they facilitate the transmutation of fertile material into fissile material and, in some configurations, are capable of breeding fissile material. SFR, LFR, GFR and MSR systems are in this category. These characteristics could lead to improved use of energy resources. Furthermore, high-temperature coolants can be used to improve the energy efficiency of nuclear power plants as well as nuclear facilities, such as the VHTR, that could be used to generate process heat for industry. French industry has opted for the development of a Generation IV reactor prototype based on the SFR concept by 2020, via ASTRID, the Advanced Sodium Technological Reactor for Industrial Demonstration project. ASN, the French Nuclear Safety Authority, stressed that the choice of one system over another selected by GIF should be based on demonstrated safety and radiation protection performance. In this respect, it considers that the system selected for the development of Generation IV reactors in France should provide significantly more protection for the interests mentioned in Article L593-1 of the French Environmental Code1 than currently offered by 1 Public security, health and safety, protection of the nature and the environment 2/239 Generation III reactors2. ASN therefore wished to obtain the opinion of the Advisory Committee for Reactors (GPR) on the safety and radiation protection characteristics of the six systems studied by GIF. IRSN would first like to draw attention to the difficulties of performing a “balanced” safety and radiation protection review of the nuclear systems selected by GIF, as some concepts have already been partially tried and tested, while others are still in the early stages of development. Consequently, some technologies benefit from substantial operating experience feedback and in-depth studies, while others are only at the project stage. The safety of a nuclear facility depends both on the intrinsic characteristics of the facility in question and the design and operating provisions implemented. For that reason, it is impossible to make a comprehensive assessment of the different systems at this point in time and designs currently under consideration may not necessarily be those ultimately selected for Generation IV reactors and the related fuel cycle. IRSN’s assessment thus seeks to measure the “safety potential” of the systems in question as far as can be determined on the basis of current knowledge. To this end, IRSN focused on reviewing the intrinsic characteristics of each system and the related design and operating constraints, including aspects specific to fuel cycle facilities. In particular, it considers that the future generation of nuclear facilities would benefit from the development of “tolerant” concepts that are not highly sensitive to events liable to occur inside or outside the facility. Sodium-cooled Fast Reactors (SFR) Based on its assessment, IRSN considers the SFR system to be the only one of the various nuclear systems considered to date to have reached a degree of maturity compatible with the construction of a Generation IV reactor prototype during the first half of the 21st century. Furthermore, the scenario in which PWRs would be phased out and replaced by SFRs by the end of the century is plausible given the ready supply of plutonium in the initial stages of the deployment scenario and the use of a closed nuclear fuel cycle with proven oxide fuel. Before this scenario can be rolled out, however, further progress must be made in research and technological development in areas that are, for the most part, already identified. Metal fuel is another option that could be considered, albeit it in the longer term as its use would involve extensive changes to current reprocessing methods and require a significant R&D effort. Investment on this scale would only appear justified if the use of metal fuel brought about a significant improvement in reactor safety. The main advantage of SFR technology in terms of safety is the use of low-pressure liquid coolant. The normal operating temperature of this coolant is significantly lower than its boiling point (margin of about 300°C), allowing a grace period of several hours during loss-of-cooling events. The advantage gained from the high boiling point of sodium, however, must be weighed against the fact that the structural integrity of the reactor cannot be guaranteed near this temperature. The use of sodium also comes with a number of drawbacks due to its high reactivity not only with water and air, but also with MOX fuel. Design provisions must be made to mitigate the related risks. While it seems possible for SFR technology to guarantee a safety level at least equivalent to that targeted for EPR- type pressurised-water reactors, IRSN is unable to determine whether it could significantly exceed this level, in view of design differences and the current state of knowledge and research. The nuclear system associated with the SFR has been the focus of considerable R&D, both in France and elsewhere. One example is the ASTRID reactor 2 In other words, meeting safety objectives on a par with those defined for the Flamanville 3 EPR currently under construction. 3/239 should lead to a more accurate assessment of the various technological solutions studied in terms of feasibility and safety. Very High Temperature Reactors (VHTR) The VHTR benefits from the operating experience feedback obtained from High Temperature Reactors (HTR). Although it aims at higher technical performance levels than the HTR, it does not offer any progress in terms of safety. Nonetheless, the safety level of the last HTRs developed in the 1980s is already an improvement compared with the other systems selected by GIF. In particular, this technology is intrinsically safe with respect to loss of cooling, which means that it could be used to design a reactor that does not require an active decay heat removal system. The VHTR system could therefore bring about significant safety improvements compared with Generation III reactors, especially regarding core melt prevention. The feasibility of the system, however, has yet to be determined and will chiefly depend on the development of fuels and materials capable of withstanding high temperatures; the currently considered operating temperature of around 1000°C is close to the transformation temperature of materials commonly used in the nuclear industry. A precise assessment of risks relating to graphite dust would also be required if this option was selected. As it stands, this system does not optimise natural resource and waste management in the long term; storing structural waste and spent fuel without conditioning is not a viable permanent solution. Alternative management solutions are being explored, however, to limit the quantities of graphite for disposal. Although it was contemplated in the past, the use of a closed fuel cycle for the VHTR system is not feasible at present. Demonstrating that VHTR waste can be safely managed and determining the long-term behaviour of waste after disposal (and defining how it should be conditioned) are therefore two key factors in decision-making. The closed cycle option could, however, become more credible in the longer term if thorium fuel (233U-Th) were to be used, although substantial R&D would be required to demonstrate feasibility of the concept. Whatever the case, managing this type of fuel cycle would be a long and complex task. It is clear that VHTR safety performance can only be guaranteed by significantly limiting unit power, which makes it very unlikely that VHTRs will one day replace French nuclear power plants. In view of the above, IRSN sees the VHTR more as a way of consuming plutonium to back up the PWR fleet, for example, and reducing plutonium inventory in the fuel cycle - even significantly so within the context of a nuclear phase-out. No operating experience feedback from the other four systems studied can be put to direct use. The technological difficulties involved rule out any industrial deployment of these systems within the time frame considered. Nevertheless, a distinction can be made between the LFR and GFR systems on the one hand, for which small reactors could be built during the first half of this century, and the MSR and SCWR systems on the other, where it seems hard to imagine any reactor being built before the end of the century. 4/239 Lead-cooled Fast Reactors (LFR) Lead has some useful neutron properties and, unlike sodium, does not react violently with water or air. The thermal inertia associated with the large volume of lead used and its very high density results in long grace periods in the event of loss of cooling.